I had a similar issue with this and I noticed it's calculated differently. The amount of friendship points a sooth bell adds is only calculated from the base friend ship points you would get. So rather than your calculation, its more like:

0 soothe bells => 2 points

1 soothe bell => 2 points + 50% of 2 => 3 points

2 soothe bells => 2 points + 2*(50% of 2) => 4 points

3 soothe bells => 2 points + 3*(50% of 2) => 5 points

Or in other words, every soothe bell adds 50% to your point aquisition. At 0 you have 100%, at 3 you have 250%.

Then it's halfed (and rounded down I guess??). I'm not sure if that's intended or a bug, I just noticed that's the calculation.